The geometry of flux surfaces with quasi-poloidal symmetry

Quasi-poloidal (QP) magnetic fields have desirable properties for confining plasma: no radial drift of guiding centres (with positive implications for neoclassical transport), zero Pfirsch–Schlüter current and a lower level of damping for poloidal flows. Despite their attractive properties, QP fields are not amenable to the near-axis expansion, a major theoretical tool for understanding toroidal fields. In this paper we provide a novel framework for defining and understanding QP flux surfaces. This framework relies on a simplification that transforms the task of finding a QP flux surface from a three-dimensional problem to a two-dimensional (2D) problem. This simplification also applies to asymmetric magnetic mirrors with desirable properties. We sketch how this 2D problem can form the basis of an efficient optimisation problem for finding QP flux surfaces. We leverage this 2D problem for theoretical understanding: for instance, we identify a route to finding QP flux surfaces that are naturally flat mirrors (Velasco et al. 2023, Nucl. Fusion , 63, 126038). The reduced model is qualitatively checked against numerically optimised QP equilibria. These numerical solutions only satisfy QP approximately, but we predictably find that local discrepancies with the reduced model correspond to significant local QP errors, anomalous parallel currents and field lines deviating from geodesics.